1. Field of the Invention
The present invention related generally to communication of digital information and more specifically to channel state information estimation employed in decoding transmitted digital information.
2. Description of Related Art
Digital information is transmitted from one place to another through a channel. A channel is a general term referring to the medium through which a signal is transmitted. The channel may be a radio transmission, local area network transmission, telephone transmission, etc. In each case, the received signal differs from the transmitted signal due to the effects of the transmission through the channel. In most digital communication systems it is desirable to have knowledge of this channel information, known as channel state information (CSI) at any instant of time in order to properly decode the transmitted information.
Typically, CSI is derived using synchronization ("sync") signals, which may be sounding signals or a set of pilot symbols sent by the transmitter. The sync signals are predetermined and stored in the receiver. During transmission, the receiver compares the stored sync signals with the received signals and estimates the channel impulse response (CIR). This may be performed by a mean squared error or a least squares error formulation.
The received signal may be viewed as a vector comprising an amplitude component and a phase angle. In many applications only the phase angle of the received signal is retained for decoding the signal, as in the case of differential detection. The motivation for retaining only the phase angles is that the receiver may be is simplified. Also, with differential encoding and detection, the transmission is more resistant to fading and Doppler frequency shifts. In other situations the received signal may be hard-limited, where the amplitude is limited to a maximum amplitude value, and hence only the phase angles of the received signal are available for decoding. The transmitted signals is sent with message data interleaved in periods between the sync signals. Therefore, in the absence of known sync signals (pilot or sounding), the CSI must be estimated from previous CSI or determined from the received signal directly.
CSI may be used in a number of ways to enhance detection of the transmitted signal. Some examples are:
Post-Detection Selection Diversity for Radio Transmission PA0 PLL Adaptation PA0 Errors and Erasure Decoding
A typical implementation of post-detection selection diversity will employ several antennae each attached to a separate receiver branch. The data is demodulated independently for each receiver branch to produce an output signal. The CSI of each output signal is determined and the output signal having the best CSI is chosen for symbol decoding. The decision for choosing which one of the receivers output signals to decode may be performed on a symbol- by-symbol basis.
In a coherent communication system it is customary to use a circuit which tracks and adjusts to carrier frequency changes, commonly known as phase-locked loops (PLLs). The PLL makes adjustments based upon perceived error. If the channel quality is poor, incorrect updates are fed to the PLL and it can become unstable. This can be prevented by using CSI information to adjust the PLL adaptation according to channel conditions. A similar argument holds for phase adjustment loops (PALs) employed in maximum likelihood sequence estimation (MLSE) decoders which track the carrier phase.
Errors and Erasure decoding is typically employed with forward error correction schemes. Each decoded bit or symbol is identified as good or corrupted. The corrupted bit/symbols are marked as such. A good bit/symbol is detected as is, but an corrupted bit/symbol is ignored. The redundant error correction scheme at a receiver derives a replacement bit/symbol for each corrupted bit/symbol from the remaining bits/symbols. If the corrupted bit/symbol was employed in decoding, then a mistake could have been made which in turn would cause further errors. Hence, erasure decoding enhances bit error rate (BER) performance considerably. To effect such erasure decoding a symbol by symbol estimate of CSI is required.
Currently there is a need for a suitable measure of channel quality (CSI) from the phase angle of the received signal.